There are various procedures known in the prior art for the preparation of refractory composite structures which are resistant to degradation through oxidation or applied thermal and mechanical stresses or under severe temperature conditions. Such refractory structures can incorporate the use of metal powders such as those used in powder metallurgy processes, ceramic powders and mixtures of ceramic and metal powders commonly referred to as ceramals and cermets. Such products are employed in high temperature environments up to 3000.degree. F. and even beyond as components in turbine engines and heat exchangers. They are also used in low temperature structures requiring characteristics such as high strength/weight ratios, high corrosion resistance, high erosion resistance, and high dielectric capacities. Such materials find uses in the electronics industry and in various bearing applications.
Procedures used in formulating ceramic composite structures include manufacturing processes involving cold or hot isostatic pressing, uniaxial hot pressing, injection molding, slip casting, and sintering procedures, which may or may not include liquid phase sintering. Various materials may be employed in producing refractory composites. For example U.S. Pat. No. 3,833,389 to Komeya et al discloses a composite which is reinforced by an inner fabric structure and which is derived from a multicomponent mixture comprising one or more ceramic powders. In Komeya et al, the heat resistant composite is formed from a three component mixture. The first component is aluminum nitride or silicon nitride or mixtures thereof. The second component is an oxide of lanthanum, cerium, scandium, yttrium, and/or yttrium aluminum garnate. The third component is silicon carbide, boron nitride, or carbon or mixtures thereof. The three components are mixed and then hot press sintered in a nonoxidizing environment. Komeya et al contemplate the use of fibers, for example, silicon carbide whiskers, and also binders such as steric acid which can be decomposed or otherwise vaporized out of the system during the sintering procedure.
U.S. Pat. No. 4,507,224 to Toibana discloses composite structures of high electroconductivity characterized in the patent as ceramics A and B. The A materials are based upon a matrix phase of oxides of Group II, Group III and Group IV elements which are reinforced with crystal whiskers of silicon carbide. The group B ceramics may have a matrix phase formed of the above oxides or alternatively nitrides and/or carbides of Group II, III or IV elements and also electroconductive carbides, nitrides and borides. Sintering aids which may be used include magnesia, alumina, and yttria. The materials used in forming the composite are formed into a pasty mixture which is molded to a desired shape by injection molding or extrusion and dried at temperatures up to 600.degree. C. and then fired at temperatures ranging from about 1300.degree.-1800.degree. C. for the A materials and 1200.degree.-2000.degree. C. for the B materials.
U.S. Pat. No. 4,543,345 to Wei discloses a ceramic composite and its method of preparation in which monocrystalline silicon carbide whiskers are used to reinforce the composite material based upon refractory matrix powders such as Al.sub.2 O.sub.3, 3Al.sub.2 O.sub.3.2SiO.sub.2, and B.sub.4 C. The silicon carbide whiskers are characterized as having an average diameter of 0.6 microns, a length of 10-80 microns, and an average aspect ratio (the ratio of whisker length to whisker diameter) of 75.
Wei discloses two general procedures for forming the composite. The first to produce a product in which the whisker orientation is in a plane orthagonal to a pressing axis is exemplified by the procedure in which fine ceramic powders (0.5-1.0 micron) and silicon carbide whiskers are mixed in hexane and then agitated in a blender followed by dispersion in an ultrasonic homogenizer. The resulting mixture is dried and then hot pressed to a density of more than 99% of theoretical density. Hot pressing is carried out at temperatures of 1600.degree. to 1950.degree. C. and pressures of 28-70 MPa. An alternative to the use of hexane as a solvent in this procedure is distilled water which is removed by freeze drying prior to the hot pressing step. An alternative procedure designed to achieve omnidirectional whisker orientation involves isostatic hot pressing. Here the pressures and temperatures applied to the mixture in a tantalum can in a high temperature inert-gas autoclave are in the some ranges as those employed in the uniaxial pressing procedures.
U.S. Pat. No. 4,560,668 to Hunold et al discloses the production of shaped composites based upon mixtures of polycrystalline silicon nitride and polycrystalline silicone carbide powders having particle sizes up to 10 microns. The particulate mixture is mixed with a temporary binder and dispersed in a solution of a solvent such as acetone or a C.sub.1 -C.sub.6 aliphatic alcohol and then shaped by known technique such as die pressing, isostatic pressing, injection molding, extrusion molding or slip casting. After the shaping procedure, which is carried out at room temperature or above, the shaped green composite is heated to a temperature from 300.degree. to 1200.degree. C. prior to an encapsulated isostatic hot pressing procedure. The thermotreatment is employed in order to ensure that gaseous decomposition products from the binders do not interfere or damage the casing employed in the hot isostatic pressing process. The composite materials enclosed within a suitable casing such as tungsten, glass, etc. are heated in a high pressure autoclave at temperatures within the range of 1800.degree.-2200.degree. C. at pressures of from 100 to 400 MPa.
U.S. Pat. No. 4,652,413 to Tiegs discloses a process for producing silicon carbide whisker reinforced refractory oxide composites in which pressureless liquid phase sintering is followed by hot isostatic pressing. In this procedure, a mixture of a particulate matrix powder such as Al.sub.2 O.sub.3, 3Al.sub.2 O.sub.3.2SiO.sub.2, B.sub.4 C is mixed with about 5-60 volume percent of SiC whiskers of about 0.6 microns in diameter and about 10-80 microns in length. Specifically disclosed in Tiegs is a mixture of alumina powder in the size range of about 0.1-1 micron, about 0.5 to 5 wt. % of yttria in the size range of about 1 to 3 microns as a liquid phase sintering aid and about 5 to 10 volume percent silicon carbide whiskers. The components are mixed and then subjected to wet milling in a ball mill operation for a period of about 1/2 to 8 hours. This mixture is then pressed in a suitable mold under a pressure of 10,000 to 60,000 psi to at least 50% of theoretical density and is then subjected to a pressureless liquid phase sintering procedure at a temperature of 1800.degree. C. in an inert gas atmosphere provided by argon, nitrogen or helium. The resulting product is then subjected to a hot isostatic pressing in an argon atmosphere at a pressure of 10,000 to 30,000 psi and a temperature of 1600.degree.-1700.degree. C.